Why Is It So Hard to Define a Species?

The “species” category is almost certainly the best known of all the taxonomic classifications that biologists use to organize life’s vast diversity. It’s a linchpin of both conservation policy and evolutionary theory, though in practice biologists have struggled to find a definition that works across the natural world. In this episode, Kevin de Queiroz, a zoologist and evolutionary biologist, talks with host Janna Levin about the variety of ways to conceive of a species, and ways to understand the relationships among living things.

Listen on Apple Podcasts, Spotify,  TuneIn or your favorite podcasting app, or you can stream it from Quanta.

Transcript

[Theme plays]

JANNA LEVIN: If you were to look around and start counting all the organisms in your line of sight — flora and fauna, single-celled and multicelled, macroscopic and microscopic — the task would far exceed a human lifetime. A single acre may hold hundreds of millions of individual organisms. So it’s next to impossible to say precisely how many co-inhabitants we have here on Earth.

Perhaps in an effort to determine how we fit into the mix, we’ve spent hundreds of years attempting to categorize the living things around us, grouping them by shared traits into a series of taxonomic ranks — of which the most specific, and likely most familiar, is species.

But it turns out even our most precise point of classification isn’t all that well-defined. If we don’t truly understand species, how can we possibly understand evolution, conservation or ourselves?

I’m Janna Levin. And this is “The Joy of Why,” a podcast from Quanta Magazine, where I take turns at the mic with my co-host, Steve Strogatz, exploring the biggest questions in math and science today.

In this episode, we take a look at what “species means,” why it’s so hard to pin down, and why it’s important to get that definition right.

[Theme fades out]

We’re joined by Kevin de Queiroz, a research zoologist and curator of amphibians and reptiles for the National Museum of Natural History of the Smithsonian Institution. He’s been central in the development of a unified concept of species, using an approach that focuses on phylogenetics, which is to say the evolutionary history of species and relationships between groups of organisms.

Kevin, it’s great to have you on “The Joy of Why.”

KEVIN DE QUEIROZ: Thanks very much, Janna. It’s a pleasure to be here.

LEVIN: You know, we can’t really talk about species without talking about [Charles] Darwin and without talking about Darwin’s On the Origin of Species. Can you tell me a little bit, get us anchored in Darwin’s thoughts on the topic?

DE QUEIROZ: Yeah, well, Darwin was actually a very important figure in the development of a modern species concept because he proposed evolution, and specifically evolution by natural selection. He kind of reoriented the way people thought about species because before people had this evolutionary worldview, they viewed species as more like separate acts of creation.

And how Darwin changed that is, he talked about species as what he called “branches in the lines of descent.” And he actually diagrammed that in The Origin of Species, using a diagram that’s not unlike the sort of things we call “phylogenetic trees” today that show how species are related to each other. And so, for him, species were these branches in the lines of descent.

LEVIN: Now, you can have these branches, like you’re saying, in the line of descent. We’re all kind of familiar with that tree, that evolutionary concept of things radiating off of some kind of common source. But at what point do you call the branches different species?

DE QUEIROZ: That’s actually a big part of the problem about modern notions about species. Darwin, he was really looking at two things.

One is gaps. Like, if two things look different, but you could find intermediate gradations between them, that would be evidence against them being different species.

But if there weren’t gaps, then it was the amount of difference that determined when he would call them different species. But of course, that’s a very subjective, arbitrary cutoff, and different naturalists might draw that line in different places.

LEVIN: Now, so far, I have a sense that, you know, here comes Darwin, these fascinating descriptions of common ancestry and things branching off. Incredible observations; he used to draw, of course. A lot of these things. And even after all these years, you know, that was the mid-1800s, I’m not hearing a really crisp definition of species yet. Do you feel we have one?

DE QUEIROZ: Yeah, so if we want to go back to the definition of species, you know, as I said, Darwin had this notion of these separate branches in the lines of descent. And he would consider them to be species when they reached a certain level of presumably morphological difference. OK, and that sort of idea was perpetuated.

But toward the middle of the 20th century, people started proposing different cutoffs that were, in many cases, either more objectively defined, and sometimes more relevant to the processes that actually maintained separation between these different branches — what we now call lineages.

So basically what happened was, people proposed alternative definitions. Ernst Mayr, for example, he was one of the main architects of this modern synthesis, and probably the one who wrote the most about species and species concepts. And for him, they had to be intrinsically reproductively isolated. Once the lineages reached that point where they couldn’t or wouldn’t interbreed under natural conditions, that’s when, he said, now they’re species.

But then other people proposed other cutoffs related to different biological properties.

LEVIN: Like what other different properties?

DE QUEIROZ: There were actually a lot of them. And usually, what it amounted to was people would come up with different criteria, or different sort of cutoffs. Instead of… like for Darwin, some vague amount of difference, right? They would come up with some other threshold or property that the lineage had to cross before they would call it a species.

Some of the earliest criticisms were from this school of taxonomy that was called numerical taxonomy, or phenetics. And what they complained about is, “Well, we really don’t know what things are interbreeding with each other. So, all we can really go on is, you know, similarities and differences.” And they had developed more sophisticated numerical methods for summarizing the similarities and differences that you might measure or count, rather than, in Darwin’s day, a more sort of qualitative assessment.

I don’t know if they ever came up with a specific threshold of how different they had to be by application of their numerical techniques. Other people — for example, Leigh Van Valen of the University of Chicago, he said, well, they had to reach the point where they occupy different adaptive zones.

LEVIN: Can you explain to me what an adaptive zone is?

DE QUEIROZ: We really usually talk about a “species niche” now. For Van Valen, he was studying oak trees and maybe some of them were more adapted to wetter bottomland conditions than the other ones, which were adapted to living more on the drier hillside, something like that. So that would be a difference, for him, in their adaptive zones.

LEVIN: So what is your current best definition of a species?

DE QUEIROZ: Well, so I like to use a very general definition. It’s very similar to Darwin’s branches in the lines of descent, but in sort of more modern terminology. What I say is, there’s segments of separately evolving metapopulation lineages.

The main reason that I came up with that was because there were all these different definitions of species and many of them distinguish themselves from each other by picking another property of those lineages as something that you had to have before you were considered a species. I suggested that we have just this very general notion of what a species is.

I tried to emphasize that if you really look at all these definitions, they all have a common, more general notion of what a species is. That it’s this separately evolving lineage. And then they throw in a different thing, this cutoff point.

And so what I suggested is we should just get rid of all those secondary cutoff points, and say all these things are species, and then these are properties that they have.

And there are other reasons for that too. Because when we use the cutoff point like that, it really decreases the generality of the term, and makes it sort of a less big important concept in biology. If you think of the concept of the organism, right? That’s another really basic unit of biology.

But we don’t use those same kind of cutoffs for organisms. We don’t say, “Oh, you’re not an organism until you’re born,” or “You’re not an organism until you’re sexually mature.” But that’s basically what we were doing with species. And so I suggested that we should not add in those secondary criteria and just say all of these separately evolving lineages are species.

And these other properties that they acquire as they diverge from one another, those are things that we can use, one, as lines of evidence that they’re different from each other. And two, as the basis for different subcategories of the category species. Just like we talk about an adult organism or a post-embryonic organism, we can talk about a reproductively isolated species or an ecologically differentiated species.

LEVIN: Hmm. So, I really like this example you gave of “We don’t call something an organism only when it sexually matures,” for instance. That seems clearly to be a subcategory of the sort of natural changes within that organism’s life span.

Now, why is it so important that we have a definition of species such as the one that you propose?

DE QUEIROZ: Well, I mean, we have to communicate about them, right? I mean, that’s why we have terminology, why we have language — to be able to communicate about things.

There were people who wanted to get rid of the term. I was just reading a paper by this guy [Edward Bagnall] Poulton in 1904, I think, and he was talking about how one of his colleagues argued that Darwin didn’t really explain the origin of species. He basically explained why we should get rid of species, right?

LEVIN: Hmm, because we’re all ultimately connected?

DE QUEIROZ: Because the original concept, or the concept that was in existence before him, was a very non-evolutionary concept. And so, what he was talking about, I think this guy, [E. Ray] Lankester, thought, that’s a completely different thing. We shouldn’t even call it species.

But the thing is, it’s hard to get people to get rid of these really widely used terms. And so I think what happens more often than getting rid of them is we redefine them in a way that makes them more relevant to the current theoretical context.

LEVIN: And is it getting traction? Are people dropping these notions of specific turn-on in behaviors or specific details of reproduction or jawbones or whatever?

DE QUEIROZ: You know, I haven’t done a survey to see how widely accepted it is. The paper’s been cited a fair number of times. The original term I used was “a general lineage concept of species.” That was to refer to the fact that all the species definitions had this in common.

LEVIN: And how does your approach deal with hybrid species, which I’ve come to understand are notoriously difficult to classify in some of the other schemes?

DE QUEIROZ: Yeah. Well, if you just said hybridization between the members of two species and their descendants were infertile or something, then that wouldn’t really be a species. It’s only a species if that new thing that’s formed from the crossing between the two species forms its own separate thing.

And if it does that, I mean, that fits well into the lineage concept. It’s just that instead of the typical thing of one lineage splitting into two or more, this time you have two sort of joining together to create this other one — but not fusing together because they maintain their separateness from this hybrid species.

LEVIN: And what about issues of conservation? For instance, you were describing what I learned growing up, which is that species were defined on whether or not they could procreate. And so, things like brown bears and polar bears were considered distinct species, and yet we see that they can procreate and there’s evidence in their DNA that they do. So does that matter in terms of how we call them? I can still know that a brown bear is different than a polar bear, but it might matter if I’m concerned about conservation of species.

DE QUEIROZ: Well, I think the thing is that sometimes these, sort of, handy definitions that we learned out of textbooks, they can often be maybe oversimplified. And so just the fact that things can, you know, interbreed to produce viable offspring is not the same as them doing that frequently in nature.

So this is even something that Darwin was well aware of, that they could be separated by various sorts of things like living in different habitats, breeding at different times of year, just their courtship behaviors. You know, if they differed in those things, they would often not interbreed with each other. But that doesn’t mean never.

So, we’ve always known that some things that are pretty distinct from each other can interbreed and produce viable offspring occasionally, but they don’t do it commonly, so I don’t think anyone has trouble calling those different species.

LEVIN: We’ll be right back after this message.

[Break for ad insertion]

LEVIN: Welcome back to “The Joy of Why.”

So, how drastically has our understanding of DNA and gene history changed the concept of a species?

DE QUEIROZ: So, like all these things, they sort of emerge gradually. Darwin did have an idea about inheritance, but he didn’t have the same notion that developed later under Mendelism. But even early on, if we had this idea of dominance and recessiveness and more, sort of, qualitatively different traits, that kind of information is useful for interpreting the observations of the morphology that you have. You know, so that would be maybe an early stage in the understanding of genetics.

But of course, then people started actually seeing the chromosomes, and they made inferences based on that. And, you know, they started getting more detailed information. And then they started becoming able to sequence the DNA. And, you know, first it was just small snippets of it. Now, we’re able to sequence entire genomes of organisms.

LEVIN: And in the current theoretical context, what are some of the issues that are at play? Clearly genetics has become much more important since Darwin’s day. What else has become really influential that’s guiding these assessments?

DE QUEIROZ: Well, the basic idea hasn’t changed that much. What’s changed more is the type of data that we can collect relevant to that question and the methods that we use for analyzing those data.

People have thought of new ways in which the information that we have bears on the question, and they’ve come up with new analytical methods of what we now call species delimitation. It’s kind of determining what the boundaries and numbers of species are.

LEVIN: And the numbers are vast.

DE QUEIROZ: Oh, yeah, yeah.

LEVIN: I mean, even within the distinction of flora and fauna, let’s just take those huge categories.

DE QUEIROZ: You know, those are only a small fraction of all the organisms on Earth. They’re just the ones we think about the most because they’re big, multicellular organisms that we see all the time, right?

There are lots of other things in between them — things that branched off the lineage leading to plants and things that branched off the lineage leading to animals — that are mostly unicellular organisms.

LEVIN: So let me ask you this. At one point it was not understood, the huge numbers of species out there. When did it become clear that the number of species was just incredibly vast, much vaster than what had originally been thought?

DE QUEIROZ: You know, in the middle of the 1700s, in Linnaeus’s time, he thought one could name all the species on Earth, and it would be a small enough number that a smart person could memorize them all, right? And then we just kept discovering more and more and more, and so I’m not sure at what point people came to this explicit realization that it’s a vast number.

LEVIN: Mm-hmm. You mentioned Carl Linnaeus, a Swedish biologist. When was Linnaeus’ day?

DE QUEIROZ: It was like a hundred years before Darwin. He published his great works multiple times, but the one that’s used kind of as the benchmark in zoology is from 1758.

LEVIN: Oh, way back. So, would it be a simple question to answer, how many species of mammals are there, for instance? Is that an easily answerable question?

DE QUEIROZ: Yes and no. So, if we restricted ourselves to living mammals, that’s definitely going to make things easier. And it might be the case that we might not discover all the living mammals too, but it’s at least maybe feasible to think that we could.

This is another thing where the way we’ve tended to think about that has changed over time.

In Linnaeus’ day, we only knew about the extant mammals. And we didn’t even know about some of the ones that were at some point considered, you know, sort of ambiguous as to whether they were mammals, like when echidnas and platypuses were discovered.

You know, the name came from the fact that they nurse their young — mammals, right? And that’s probably then why they decided ultimately that echidnas and platypuses were mammals because they did nurse their young, but they did not carry them to term in utero — they lay eggs.

LEVIN: Fascinating.

DE QUEIROZ: Sort of into the 20th century, people, when they discovered fossils, it was always this big question about if it was something that was on the stem lineage of mammals. Is this a mammal or isn’t it? And at what point do we call it a mammal?

And so because it was often very difficult, if not impossible, to determine if they nurse their young, they would focus on other characters, like whether it had a single bone in the lower jaw — earlier ancestors of mammals had multiple bones in the lower jaw. So they would focus on characters like that.

But the group that I’m part of, that’s trying to develop this different naming system, we sort of have reoriented the way at least we think about that, which is to think of, like, what’s the most useful place to put that name “mammal”?

And so we like to put it at the clade of the living mammals. And we call it the “mammalian crown clade.” So clades are typically composed of multiple species, right? You would have an ancestral species and all the things that descended from it, all those things would be in the same clade.

So, it’s basically the last common ancestor of platypuses and humans and all of its descendants. Those are mammals for us.

LEVIN: So we’ve discovered a lot of new species. It seems like all the time, I’m hearing that they looked deeper in the ocean, they looked under a rock, and lo and behold, they found new species. Will that eventually ever kind of taper off, or do you think that’s just going to keep going, that the numbers are so high — the hundreds of millions of different species — that we will always find another rock to overturn to make discoveries of large numbers of new organisms. Well, not new, but new to us.

DE QUEIROZ: New to us, yeah. Yeah, I think, you know, if you look at it, it sort of varies group by group.

Some groups are both more attractive to humans, for whatever reason, and more easily studied by them. And in those groups, they’ve already tapered off, right? It doesn’t mean we haven’t found every last one of them. It’s just that the rate of new species discovery has really slowed down. And that’s true for mammals and birds.

For other groups, it hasn’t tapered off as much, but at least potentially could. But there’s some things that if and when it does, it could be a long time in the future. In particular, the unicellular organisms — not easy for us to see and they’re everywhere. And also things that live, for example, in the deep ocean that are not as easy to study. So, I imagine the rate of species discovery in those groups in those areas will continue to be high certainly for my lifetime.

LEVIN: Mm-hmm. Now, I have a question on categorization of species in human evolution. So modern human beings — you know, what we consider ourselves to be — it’s come out that we’ve mixed with Neanderthals, for instance, much more than people thought even just a couple decades ago. We can have between 1 and 4% Neanderthal DNA? And one of the smartest people I know brags about he has 3.5% Neanderthal DNA. How do you just think about that, just as a curious person?

DE QUEIROZ: Well, I think a lot of species have hybridized with other species at times and have received some, you know, genetic material from those other species. And if it’s persisted, it’s probably because there was some advantage to it, right? I don’t know, I think, you know, one of the things that I think maybe we’ve tended to do in this case of Neanderthals is, we’ve just sort of assumed that they were just universally inferior, sort of, subhuman or something to us. And so it is almost like, well, if we have material from them that that must be bad or something. But I don’t buy that.

I think they’re getting more and more evidence that Neanderthals actually had fairly sophisticated culture, and certainly for that time, the things that are more clearly related to us as modern humans, you know, they weren’t all that sophisticated either way back then.

LEVIN: [laughing] Some of them still aren’t, yeah.

DE QUEIROZ: Yeah, and so I think, you know, like your friend bragging about him having this Neanderthal DNA, I think that’s a way of, it’s similar to what I’m saying, it’s to say, “Hey, you know, that’s not necessarily a bad thing.”

LEVIN: They weren’t to be dismissed, those Neanderthals.

DE QUEIROZ: They weren’t brutes with no intellectual capability.

LEVIN: There might even be cave paintings from Neanderthals, I don’t know if that’s verified, but totally fascinating. [Editor’s note: There are indeed paintings by Neanderthals.] So, what do you think right now is the most interesting direction at this moment in your field?

DE QUEIROZ: Yeah. So, you know, we talked about DNA and how now we can sequence DNA, we can sequence whole genomes of organisms. And now the question is, how do we analyze those data to make inferences about the numbers and boundaries of species?

And I think back in the old days, you know, when people were first starting to sequence DNA, they would take the DNA sequences and generate, say, a phylogenetic tree from it. Right? And they would basically say, “Well, we know that these genes form their own phylogenetic trees at the gene level. And we know that those aren’t really the same as the species tree, but they kind of — well, we think they kind of more or less mirror what the species tree is. So, we’ll just kind of assume that they’re more or less the same.”

But they weren’t the same. And so, at some point they developed then a theory that more distinctly separated them to say that you have these gene trees evolving within a species tree. The gene tree and the species tree are not the same thing. One is kind of evolving within the other one. This is now called “coalescent theory.” We call it “coalescent” because if you had two alleles, if you trace them back in time, when do they coalesce in a common ancestor, right?

So now they have methods that are based on this coalescent theory that are more biologically realistic, that make that distinction, and then make your inferences both about phylogenetic relationships but also about species limits — incorporating that explicitly into the model.

But at least from my perspective, a lot of these methods are still fairly crude, in that they make some simplifying assumptions that not only do we think they’re oversimplified, but we also think that they could have a strong effect on the inference. That they could lead us to overestimate the number of species, for example.

And so I think the development of those methods is a very exciting field, and one where I expect to see some important improvements in the next 10 years.

LEVIN: I have heard that in this time where people are very concerned about a climate crisis, and people are talking a lot about conservation, that these issues of defining species have greater impact politically now.

So if there’s a penguin that’s considered the same species as one from a very different geographical area with a lot of the variations you were talking about, where that population’s doing well, but this other penguin in their natural habitat is not doing well — calling them the same species takes them off the endangered species list. Have you seen that as being a problem? That these definitions of species are making or breaking their survival of certain animals?

DE QUEIROZ: Well, I mean, that’s why this concept of species has been so important. It’s because species are used as units of comparison in all different aspects of biology and even in various policy things like conservation.

But one thing I would say though is that because we focus less on external visible traits for determining what species are, we tend to recognize more species now than we did in the past. And I think in cases like what you were talking about, it might be more likely that people find that those two populations were actually different species. And that might aid in the conservation of the one that was more susceptible than the other one. It’s very much a case-by-case basis.

LEVIN: You know, I’m wondering from what you’re telling me if it’s possible that there isn’t a really firm scientific definition of species. Is it possible that it just will never be that crisply defined?

DE QUEIROZ: I don’t think it’s so much that you can’t come up with an unambiguous definition. It’s that when you apply the definition in practice, there will always be these intermediate cases. That in itself isn’t even a problem. It’s more a problem for us wanting to pigeonhole them, right? If we wanted to say, “These are definitely different species,” or “These are definitely the same species.” For many things we can do that, but there are always some cases where it’s just not that clear-cut. If we’re willing to live with that, then it’s not a conceptual problem. It’s just a problem of the sort of continuous nature of nature.

LEVIN: Mm hmm. Well, Kevin, there’s a question we like to ask here at “The Joy of Why,” and that is: What about your research brings you joy?

DE QUEIROZ: Putting things together, either conceptually or empirically. Like discovering a new species or something that’s an interesting thing and might bring me joy.

Also, like with this species problem, figuring out what all these previous proposals had in common and how you might be able to reconcile them — that’s something that I enjoy. Also, I would say if you have certain colleagues who you sort of mesh with and create a synergistic relationship, where what you can come up with together is a lot more than just the sum of what either of you could have come up with alone. I think that’s a very enjoyable thing to me.

LEVIN: Yeah, scientists are very collaborative. It is a pleasure of the job. Kevin, thanks so much for joining us. Pleasure to talk to you.

DE QUEIROZ: You’re most welcome. It’s my pleasure as well.

[Theme plays]

LEVIN: Thanks for listening. If you’re enjoying “The Joy of Why” and you’re not already subscribed, hit the subscribe or follow button where you’re listening. You can also leave a review for the show — it helps people find this podcast.

“The Joy of Why” is a podcast from Quanta Magazine, an editorially independent publication supported by the Simons Foundation. Funding decisions by the Simons Foundation have no influence on the selection of topics, guests or other editorial decisions in this podcast or in Quanta Magazine.

“The Joy of Why” is produced by PRX Productions. The production team is Caitlin Faulds, Livia Brock, Genevieve Sponsler and Merritt Jacob. The executive producer of PRX Productions is Jocelyn Gonzales. Morgan Church and Edwin Ochoa provide additional assistance. From Quanta Magazine, John Rennie and Thomas Lin provided editorial guidance, with support from Matt Carlstrom, Samuel Velasco, Arleen Santana and Meghan Willcoxon. Samir Patel is Quanta’s editor in chief.

Our theme music is from APM Music. Julian Lin came up with the podcast name. The episode art is by Peter Greenwood and our logo is by Jaki King and Kristina Armitage. Special thanks to the Columbia Journalism School and Bert Odom-Reed at the Cornell Broadcast Studios.

I’m your host, Janna Levin. If you have any questions or comments for us, please email us at [email protected]. Thanks for listening.

[Theme fades out]